A reservoir in which buoyant forces keep hydrocarbons in place below a sealing cap rock. Reservoir and fluid characteristics of conventional reservoirs typically permit oil or natural gas to flow readily into wellbores.
An umbrella term for oil and natural gas that is produced by means that do not meet the criteria for conventional production.
The term is used in reference to oil and gas resources whose porosity, permeability, fluid trapping mechanism, or other characteristics differ from conventional sandstone and carbonate reservoirs. Coalbed methane, gas hydrates, shale gas, fractured reservoirs, and tight gas sands are considered unconventional resources.
Crude oil with high viscosity (typically above 10 cp), and high specific gravity.
The API classifies heavy oil as crudes with a gravity below 22.3° API. In addition to high viscosity and high specific gravity, heavy oils typically have low hydrogen-to-carbon ratios, high asphaltene, sulfur, nitrogen, and heavy-metal content, as well as higher acid numbers.
Natural gas produced from gas shale formations.: Oil obtained by artificial maturation of oil shale. The process of artificial maturation uses controlled heating, or pyrolysis, of kerogen to release the shale oil.
Gas produced from a relatively impermeable reservoir rock. Hydrocarbon production from tight reservoirs can be difficult without stimulation operations. Stimulation of tight formations can result in increased production from formations that previously might have been abandoned or been produced uneconomically. The term is generally used for reservoirs other than shales.
Oil found in relatively impermeable reservoir rock. Production of tight oil comes from very low permeability rock that must be stimulated using hydraulic fracturing to create sufficient permeability to allow the mature oil and/or natural gas liquids to flow at economic rates.